This invention relates generally to self insulating substrate tapes, superconducting composite tapes and methods of manufacturing the tapes. In particular, the invention relates to the manufacture and use of self insulating substrate tapes to eliminate the problem of insulation for the construction of high temperature superconductor magnets, and to enable application of xe2x80x9cwind and reactxe2x80x9d processes with insulation away from the high temperature superconductor phase.
In the manufacture of high temperature superconductor magnets, it is often desirable to use composite tapes employing high temperature compatible insulating materials. Specifically, pancake coils are wound from high temperature superconductors in which a xe2x80x9cwind and reactxe2x80x9d approach is used due to the brittle nature of the materials. As a result, it becomes necessary to use high temperature compatible electrically insulating materials for turn-to-turn insulation in any magnet built from the high temperature superconductors. In the past, ceramic based papers or tapes have been used for this purpose. However, a disadvantage with this type of approach is that such insulation, of course, takes up valuable winding space, and currently available materials have a thickness of about 0.1 mm or greater. In addition, insulating materials are in direct contact with the superconducting materials when magnets are made from dip coated high temperature superconducting (HTS) tapes. Almost all of the insulating materials react with the HTS materials (especially BSCO) during the final partial melt heat treatment process.
The wind and react method of the prior art involves winding the precursor to a superconducting material around a mandrel in order to form a coil, and then processing the coil with high temperatures in an oxidizing environment. The processing method results in the conversion of the precursor material to a desired superconducting material, and in the healing of micro-cracks formed in the precursor during the winding process, thus optimizing the electrical properties of the coil. The superconducting coils, like most coils, are formed by winding an electrically insulated conducting material around a mandrel defining the shape of the coil. When the temperature of the coil is sufficiently low that the conductor can exist in a superconducting state, the current performance of the conductor is increased and large magnetic fields can be generated by the coil.
As is well known, certain ceramic materials exhibit superconducting behavior at low temperatures, such as the compound Bi2Sr2CanCun+O2n+4 where xe2x80x9cnxe2x80x9d can be either 0, 1, and 2. One material, Bi2Sr2Ca2CU3O10 (BSCCO(2223)), has performed particularly well in device applications because superconductivity and corresponding high current densities are achieved at relatively high temperatures, Tcxe2x89xa1115xc2x0 K. Other oxide superconductors include general Cuxe2x80x94O-based ceramic superconductors, such as members of the rare-earth-copper-oxide family (i.e., YBCO), thallium-bariumcalcium-copper-oxide family (i.e., TBCCO), the mercury-barium-calcium-copper-oxide family (i.e., HgBCCO), and BSCCO compounds containing lead (i.e., Bi, Pb)2 Sr2Ca2CU3O10).
Electrically insulating materials surrounding the conductors are used to prevent electrical short circuits within the winding of a coil. From a design point of view, the electrical insulation layer must be able to withstand large electric fields (as high as 4xc3x97105 V/cm in some cases) without suffering dielectric breakdown, a phenomenon that leads to electrical cross-talk between neighboring conductors. At the same time, in the past, it was desired to make electrical insulation layers as thin as possible (typically 50-150 xcexcm) so that the amount of superconducting material in the coil can be maximized.
By using existing conducting and electrically insulating materials, the maximum magnetic field generated by a superconducting coil is ultimately determined by the winding density (defined as the percentage of the volume of the coil occupied by the conductor) and the coil geometry. The bend strain of a conductor, equal to half the thickness of the conductor divided by the radius of the bend, is often used to quantify the amount of strain imposed on the conductor through coil formation.
Thus, instead of the xe2x80x9cwind and reactxe2x80x9d process previously discussed, one prior method used to fabricate coils with multi- and mono-filament composite conductors is the xe2x80x9creact and windxe2x80x9d process. This method first involves the formation of a insulated composite conductor which is then wound into a coil. A precursor to a composite conductor is fabricated and placed in a linear geometry, or wrapped loosely around a coil and placed in a furnace for processing. The precursor can therefore be surrounded by an oxidizing environment during processing, which is necessary for a conversion to the desired superconducting state. In the xe2x80x9creact and windxe2x80x9d processing method, insulation can be applied after the composite conductor is processed, and materials issues such as oxygen permeability and thermal decomposition of the insulating layer do not need to be addressed.
In the xe2x80x9creact and windxe2x80x9d process, the coil formation step can, however, result in straining composite conductors in excess of the critical strain value of the conducting filaments. Strain introduced to the conducting portion of the wire during the coil fabrication process, both bend strain and handling, can result in micro-crack formation in the ceramic grains, severely degrading the electrical properties of the composite conductor.
Alternatively, in the xe2x80x9cwind and reactxe2x80x9d process previously discussed, the eventual conducting material is typically considered to be a xe2x80x9cprecursorxe2x80x9d until after the final heat treating and oxidation step. Unlike the xe2x80x9creact and windxe2x80x9d process, the xe2x80x9cwind and reactxe2x80x9d method as applied to high temperature superconductors requires that the precursor be insulated before coil formation, and entails winding the coil immediately prior to a final heat treating and oxidation step in the fabrication process. This final step results in the repair of micro-cracks incurred during winding, and is used to optimize the superconducting properties of the conductor. However, these results are significantly more difficult to achieve for a coil geometry than for the individual wires which are heat treated and oxidized in the xe2x80x9creact and windxe2x80x9d process.
Due to the mechanical properties of the conducting material, superconducting coils fabricated using the xe2x80x9cwind and reactxe2x80x9d approach with composite conductors have limitations related to winding density and current carrying capability. Although the xe2x80x9cwind and reactxe2x80x9d process may repair strain-induced damage to the superconducting material incurred during winding, the coils produced are not mechanically robust, and thermal strain resulting from cool down cycles can degrade the coil performance over time. Moreover, currently available insulation takes up a lot of winding space limiting the number of turns achievable, and further limiting the teslas at the highest field achievable in the bore of a magnet.
In accordance with the invention, the problems of the prior art xe2x80x9cwind and reactxe2x80x9d approach are avoided by use and application of insulation which is significantly thinner. As such, the winding space saved can be used to result in more amp turns, allowing for an increase in teslas at the highest field achievable in the bore of a magnet.
In accordance with one aspect of the invention, there is provided a self-insulating substrate tape which includes a first and a second conducting tape, both having facing sides which face each other. An insulating layer secures the facing side of the first conducting tape to the facing side of the second conducting tape by adhering to the facing sides of the two tapes. The insulating layer is of a material capable of withstanding temperatures sufficiently high to make superconducting materials from refractory materials, or precursors, without substantially degrading the insulation properties of the layer.
The insulating layer is also typically a refractory or ceramic material, more typically alumina or zirconia powder, deposited at a thickness of about 5 xcexcm to about 40 xcexcm. More preferably, the insulating layer has been deposited using a sol-gel coating technique.
The conducting tapes are typically made of silver, nickel or nickel alloy, or silver alloy, and the silver alloy is typically one of silver magnesium alloy, silver zirconium alloy, silver aluminum alloy, or silver yttrium alloy. Further, the tapes typically have a thickness of no more than about 25 xcexcm. The tapes are preferably bonded together by rolling, or alternatively, by conventional hot rolling, which could improve bonding of the tapes and the insulating layer together.
In an alternative aspect, the invention relates to a superconducting composite tape which includes a self insulating substrate of the type described. In addition, a first layer of high temperature superconducting material or superconductor precursor to an eventually formed superconducting material is deposited on the non-facing side of the first conducting tape, as well as on the non-facing side of the second conducting tape.
In another aspect, the superconducting composite tape is wound in two-in-hand fashion as in FIG. 3 into a pancake coil. More preferably, conducting contact is established between the outermost and/or the innermost high temperature superconducting material layer with at least a next inwardly or outwardly adjacent high temperature superconducting material layer of the pancake coil, to result in a superconducting composite tape which is assembled into a coil magnet.
In yet still another aspect, the invention relates to a method of making a self insulating substrate tape. The face of a first conducting tape is coated with an insulating material capable of withstanding temperatures sufficiently high to make superconducting materials from refractory materials without substantially degrading the insulating properties of the layer. A face of a second conducting tape is similarly coated with the insulating material. The two tapes are then bonded to each other at the coated faces to result in a substrate for high temperature conductors. As in the case with the above-described tapes, the insulating material is a refractory material or ceramic material, more typically, alumina or zirconia powder.
The conducting tapes are made of silver, nickel alloy or silver alloy, and the silver alloy being preferably silver magnesium alloy, silver zirconium alloy, silver aluminum alloy, or silver yttrium alloy. The bonding is conducted preferably by rolling the conducting tapes together with the coated faces facing each other, and alternatively with hot rolling to improve bonding. The coating of the tapes with insulating material prior to bonding is done in manner in which the resultant insulating layers between both tapes is of a thickness of about 5 xcexcm to about 40 xcexcm. To achieve a thickness of about 5 xcexcm, preferably a sol-gel process is employed in applying the insulating layer.
More preferably, the invention further includes the step of coating the side edges of the self insulating substrate with a polymer coating. Thereafter, a superconducting composite tape can be made by dip coating, electrophoretic depositing or doctor blade casting of high temperature superconductor precursors on the non-coated faces of the first conducting tape and the second conducting tape. Either before winding or after winding, the assembled components can be annealed in an oxidizing atmosphere, so that the high temperature superconductor precursor material is converted to a superconducting material. The polymer coating which is used to keep the superconducting materials on either face from contacting each other, is burned off as a result of the annealing to result in clean edges of the superconducting composite tape (i.e., not shortened by the formation of superconducting phase across the edges of the first and second tapes).